The androgen receptor (AR) is a nuclear hormone receptor and promotes prostate cancer growth through activation of its downstream target genes. Although the targets of AR activation remain unclear, they are believed to be critical for cellular proliferation because most prostate cancers express the AR and are androgen-dependent. That depletion of androgens results in significant regression of prostate tumors was demonstrated in 1941 by Charles Huggins and Clarence Hodges and heralded what is now the ubiquitous strategy of androgen deprivation therapy to treat prostate cancer. Unfortunately, within two to three years after initiating therapy, most patients invariably relapse with a more aggressive form of prostate cancer, known as castration resistant prostate cancer (CRPC). There is no effective treatment option for CRPC, which has mainly contributed to the mortality of the disease. AR gene amplification has been observed in almost one-third of prostate cancers after androgen ablation therapy. Global gene expression profiling shows AR as the only gene to be consistently up-regulated in CRPC, implicating the significance of AR in disease progression. We and others have provided multiple lines of evidence demonstrating a critical role of AR in ligand-independent cell growth, implying that androgen- independent cells may still be AR dependent and that the AR can be used as a possible therapeutic target. However, the precise role of the AR in prostate cancer progression and CRPC development is still unclear. One of the main reasons for the limited progress is the lack of appropriate animal models that can be used to evaluate ligand independent AR action during the course of disease progression. To address this, we have developed several novel mouse models that allow us to investigate the progression of prostate cancer that is androgen independent but AR-dependent, mimicking what typically occurs in most human prostate cancers. In this competing renewal, we propose three different but integrated specific aims to directly test our central hypothesis that abnormal activation of AR dysregulates cell differentiation and proliferation that directly contribute to prostate cancer initiation and progression. Three specific aims are proposed in this application to address three different but related questions: 1) how abnormal activation of AR promotes tumor progression? 2) does aberrant activation of AR induce expression and activation of ETS proteins in promoting prostate cancer progression? 3) what are the molecular mechanisms underlying aberrant activation of AR in prostate cancer progression and CRPC development?